1. Field of Invention
This invention relates to an improved method and apparatus for quenching steel plates.
2. Description of the Related Art
Since the introduction to the construction industry by the steel producers of low carbon, heat treated, weldable, constructional alloy steel plates of high mechanical yield strengths and superior notch toughness, the most critical phase of the manufacturing process has been the water quenching of the plates during heat treatment.
Commercial steel quenched and tempered heat treated plates generally range in size from 20 to 50 feet long, 72 to 156 inches wide and 3/16 to 4 inches thick and in weight form ¼ to 50 tons.
The purpose of the quench is to cool a plate uniformly and sufficiently rapid, from a high temperature, usually 1650 degrees F., to obtain the optimum microstructure throughout the full thickness of the plate and to retain or improve the plate flatness as produced by the rolling mill. This operation is best explained by describing both the heat treating equipment originally used along with the modern day prior art equipment in terms of steel microstructures that are dependent upon the steel analysis, temperature and cooling rate in the quench.
The fundamental property of steel from which its response to heat treatment derives is the ability of steel to exist with its atoms arranged in two distinctly different crystallographic forms; one that is characteristic of steel at high temperature (above 1500 degrees F. for a 0.20% carbon steel); and one that is characteristic of steel at lower temperatures. The high temperature form is called austenite, and depending on its rate of cooling, will transform into either of two general categories of low temperature forms-lamellar or acicular. The acicular structure called martensite is a much stronger form of steel than the lamellar pearlite and is the microstructure desired when water quenching. The martensitic microstructure, after a tempering heat treatment, usually between 750 degrees F. and 1300 degrees F., results in a more optimum combination of strength, ductility and notch toughness than any of the known steel microstructures. Every analysis of steel has a minimum rate of cooling from the austenitic form that will result in a transformation to martensite. This rate is referred to as “Critical Cooling Rate.” Alloy additions to steel lower this rate. During the quenching operation any portion of a plate that cools slower than its critical cooling rate will transform to the lower strength lamellar microstructure. Keeping in mind that the internal portions of a plate cool slower than the plate surface and in the case of a non-uniform quench the steel analysis must be designed with enough alloy content to compensate for the slowest cooling location. Such characteristics of the quenching apparatus as cooling severity and uniformity have dictated the alloy content for any plate thickness, and the maximum thickness of plate of any specific alloy that will result in complete transformation to martensite.
The original method of heat treating quench and temper plates was to heat the plate in a car bottom furnace, lift the hot plate from the car bottom by an overhead crane equipped with plate hooks attached to a spreader bar and submerge the plate held horizontally into a water bath. The plate hooks held only a small portion of the plate edges. Steam pockets formed on the bottom plate surface, creating uneven cooling between the top and bottom surfaces causing a high degree of plate distortion. Since all these plates required mechanical leveling their strength level, width and thickness were restricted to the flattening machines capabilities. A further restriction depended on the end use of the plate, as sheared or gas cut sections from a leveled plate distorted them to a degree proportional to the amount of cold work applied to the parent plate during leveling. The first efforts to improve upon this method employed for the plate supports was a series of C shaped hooks attached to a spreader bar that lifted a plate from the furnace car bottom and submerged it into a water dip tank. This device supported a plate across its entire width and allowed for the quenching of wider plates that would otherwise buckle when lifted by plate hooks. This method also created steam pockets on the bottom plate surface and resulted in the same degree of distortion as the plate hook method.
Plates quenched and tempered by this dip quench, primarily Protective Deck Plate and Special Treatment Steel for the U.S. Navy during the World War II period were highly alloyed with nickel and chromium and presented no metallurgical problems.
Shortly after the World War II period the United States Steel Corp. developed a low alloy, high strength, quench and temper steel characterized by good weldability and toughness for the construction industry. See Hodge et al U.S. Pat. No. 2,586,042. This steel was successfully dip quenched out of car bottom furnaces but also required extensive leveling. Other plate manufacturers followed suit and there was a strong need for a quenching process that produced a flat plate. These steels were and still are produced under the specification ASTM, A514, and A517.
The maintaining of plate flatness during the quenching operation had had a far greater influence upon the design of plate heat treating facilities than the quest for microstructure. In 1942, the Drever Co. of Bethayres, Pa., a furnace manufacturer, in concert with Dofasco, engineered the first plate pressure quench to create and manufacture the continuous plate heat treating concept. In operation, a plate would exit the furnace and move into the quench on conveyor rolls. Once the plate was in position the conveyor rolls would be hydraulically lowered, the plate coming to rest on the fingers (feet, jaws) of the lower platen, while at the same time, the upper platen would be lowered onto the plate. The platens are so designed to allow space for the spray pipes that extend across the width of the plate. Water sprays from the pipes then sprayed on the plate from above and below. Typical water flows for roller pressure spray quenches range from 14 to 25 GPM/sq. ft. on each side of the plate while pressures are about 15 psi. The restraining force provided by the platens was as high as 8500 lbs/sq. ft. and as low as 350 lbs/sq.ft. on others.
In the early 1950's the Drever Co. installed the roller pressure spray quenches, as part of continuous plate heat treating facilities in plate mills world wide. The continuous plate heat treating facility consisted of a roller hearth high heat furnace for austenitizing plates, usually at 1650 degrees F., a roller pressure spray quench, roller transfer cars, roller tables for air cooling normalized plates, a roller hearth tempering furnace, for temperatures between 750 degrees F. and 1300 degrees F., and a roller leveler all in line.
To support the spray quench was a 250,000 gallon reservoir, two pumps rated at 11,000 GPM, two 24″ reverse flush Kinney type strainers, six on/off and six throttle valves, and an operating pulpit to control the system. At a later date a 200 ft. high, 1,000,000 gallon capacity water tower and additional pumps was added to ensure a more uniform water pressure to the quench. This water pumping capacity limited the length of the roller spray quench to 50 ft. To maintain the equivalent water volume and pressure for a longer quench would require additional pumping and elevated tank capacities.
The plate product quenched and tempered was the highly alloyed HY-80 and HY-100 armor plate for the U.S. Navy and the high strength commercial steel described in Hodge et al U.S. Pat. No. 2,586,042. The quenched and tempered tonnage heat treated amounted to 20% of the product, the balance being normalized carbon plate up to 3″ thick, the weight capacity of the furnace rolls.
For some twenty plus years this facility was the state of the art operating profitably at plate manufacturing mills.
During this entire time period the most serious operating problem of the facility was the choking and blocking of the spray pipe orifices with foreign material. It caused plates to be quenched out-of-flat and failures to meet required mechanical properties.
The choking was gradual at different rates at random locations and not detectable. It created a constant changing non-uniform quench pattern. Total orifice blocking appeared spontaneously and generally favored the mid-width of the quench as the spray pipes were fed from both ends. The detrimental affect increased as the hardenability (alloy) decreased and the plate thickness increased.
A quality assurance problem exists with the tendency of the pipes producing a lower quench severity to the plate mid-width and official specified mechanical tests are taken from the plate one quarter width. This created the condition that the official physical test results did not always represent the mechanical strength of the entire plate.
Analysis of the foreign material showed it to be a combination of mill scale and algae from the make-up water.
To alleviate the choking and blocking problem the plate manufacturers resorted to replacing the carbon steel spray pipes with stainless steel, installing flush valves on each pipe, reaming the pipes with rotating wire brushes and removing the pipes for redrilling. These procedures gave only temporary relief. Water treatment was also tried unsuccessfully.
In the early 1960's, the high strength heat treat plate market had become highly competitive. Plate customers were demanding the less expensive high strength steels of leaner alloys and carbon steel grades. The roller spray quenches were unable to satisfy these demands.
This brought about efforts within the steel industry to improve the quenching effectiveness of the spray quenches. It was the general opinion that the platen fingers by contacting the plate prior to the water application created a non-uniform temperature within the plate and also shielded the plate from the sprays. It was also thought that increasing the water volume and pressure would solve the problem.
The two major plate manufacturers-U.S. Steel and Bethlehem, the quench manufacturer-Drever Co. and a large plate fabricator-Caterpillar Tractor Co. initiated research programs to improve the efficiency of the plate quenches.
In the early 1960's, the Bethlehem Steel Corp. identified several of the problems and enumerated them in a paper presented at a technical meeting of the American Iron and Steel Institute. This paper described the specific disadvantages of the platens and further claimed it was also impossible to uniformly quench a stationary plate in the roller spray quench.
The Bethlehem Steel Corp. solution was to eliminate the platens and quench plates moving continuously, or progressively, between top and bottom high pressure spray curtains and then thru a lower pressure spray section similar to the roller quench. Plates were held flat between top and bottom rolls. See Malloy et al U.S. Pat. No. 3,756,869.
The Drever Co. solution was to maintain flatness between top and bottom rolls and reciprocate the plate back and forth between the top and bottom sprays. See Safford et al U.S. Pat. No. 3,423,254.
An additional solution of the Drever Co. was to quench plates moving continuously first thru top and bottom high pressure sprays and then thru the top and bottom low pressure sprays similar to the roller quench. The top and bottom rolls were tired, or ribbed, to allow better water flow on the surfaces. See Safford et al U.S. Pat. No. 3,420,083.
Caterpillar Tractor Co. also adopted the tired top and bottom rolls and higher water volumes and reciprocating the plate during the quench. See Lenz U.S. Pat. No. 3,546,911.
The aforementioned patents eliminated the platen hold down system in favor of top and bottom rolls to maintain plate flatness but retained the top and bottom sprays.
The United States Steel Corp. conducted a much more extensive in depth study to evaluate the effectiveness of their roller spray quenches. They introduced the use of the “Quench Severity Test” capable of measuring severities as close as ⅛ inch apart on the plate. The test required a Jominy Test from each plate as tested, a Rockwell C hardness test from the plate mid-gauge per test, the thermal diffusivity of the steel and the use of “Russell's Heat Flow Tables for Plates and Slabs.” The quench severities values are given in H values. H=1.00 being the value arbitrarily assigned to still water at 65 degrees F. and the historically established acceptable value for industrial plate heat treating.
Plates from the same heat were quenched on the three U.S. Steel Homestead Works plate roller spray quenches and on the Duquesne Works bar dip quench for comparison.
After being quenched the plates were tested for quench severity over the entire plate in areas under the platen fingers and in areas between the platen fingers as well as areas remote form the fingers. Brinell hardness tests were also taken in the same areas.
The results did show the platen fingers adversely affected the plate areas under the fingers. Of more metallurgical significance the results revealed the spray pattern responsible for the nonuniformity of the spray quench and the dip quench was superior to the spray quench in both quench severity and uniformity.
With the knowledge gained from their quench investigations, U.S. Steel adopted the practice of conducting periodic quench severity tests and adjusting the alloy content of their steels to compensate for low quench severity values. Low quench severity tests on their oldest No. 1 quench, installed in 1957, necessitated a complete overhaul in 1962 to remove foreign material build-up in spray pipes, operating valves, strainer screens, pipe headers and manifolds.
The Bethlehem Steel Corp. and the Drever Co. in a joint effort in 1966 designed and built at the Bethlehem Steel Burns Harbor plant, a roller quench that quenched plates moving continuously between upper and lower sprays. It consisted of a short high intensity curtain quench followed by a lower pressure quench section similar to the platen quench. Flatness was maintained between upper and lower rolls. The initial high intensity quench consisted of top and bottom water spray curtains rated at 100 psi and 260 GPM volume on each surface developed by high pressure pumps. This was followed by two high pressure spray pipes and a low pressure section, 50 ft. long, applying sprays at 25 GPM/sq ft. volume and 15 psi. on each surface supplied by the 200 ft. high water tank.
During the development phase of the continuous plate quenching process the U.S. Steel Corp. supplied different steel grades to be quenched experimentally on the Drever Co. pilot continuous quenching apparatus. U.S. Steel Corp. personnel observed the trials and evaluated the plates for flatness and quenching efficiency at their Homestead Works. The report of these trials, dated Feb. 12, 1965, shows the continuous plate heat treating process unsatisfactory in obtaining the desired quenching severity, quenching uniformity and plate flatness.
The Bethlehem/Drever quench, due to the addition of high pressure pumps, is a higher cost apparatus than the platen quenches. By 1971, the Drever Co. had installed 25 continuous quenches world wide, two of which were installed in U.S. Steel Corp. plants; one at the 160/210 inch plate mill at the Gary, Ind. Works and one at the 160 inch plate mill at Baytown, Tex. Works.
During the start up phase of these continuous quenches at both the U.S. Steel plants they failed to obtain any measure of plate flatness on any grade or thickness. Plate surface hardness readings showed unsatisfactory uniformity and hardness levels. After extensive trials under the direction of the Drever Co. personnel no improvement was made. The lighter gauge plates were severely buckled and the heavier plates were crowned across the plate width at one end dished at the other end.
U.S. Steel modified these two quenches by eliminating the high pressure sections and the top and bottom curtains and quenched plates when totally within the low pressure section. They also added time delay devices that delayed the application of water slightly to one of the surfaces with respect to the other to achieve plate flatness. See Lacy U.S. Pat. No. 4,148,673.
This U.S. Steel Corp. modification in 1972 became the state of the art. It differed from the original roller quench in that the plates were confined between the directly opposed rolls of top and bottom roller tables instead of top and bottom platen fingers and the addition of timing devices that delayed the application of water from either top or bottom sprays. Plate flatness was improved over that of the continuous quenches, but, it retained all of the disadvantages of the spray system. These quenches produced large tonnages profitably by the addition of sufficient alloy to the plate analysis to compensate for the nonuniformity and low severity of the spray quench.
An investigation was conducted on the Texas quench, on Aug. 25, 1981, to compare the water pressure on each individual spray pipe. The low pressure quench section contains 53 top and 53 bottom pipes. The pipes are fed from 3 top and 3 bottom manifolds on each side. All spray pipes were reamed by rotating wire brushes prior to the test. Readings were taken on every pipe during 3 quenches. The results showed a favorable pressure consistency for each pipe but a significant difference from pipe to pipe.
A review of the background information, as heretofore presented, and an analysis of the available technical information, reveals the specific causes of the roller spray quench limitations and shortcomings. The nonuniformity and poor quench severity values, plate flatness problems, spray pipe orifice choking and blocking, inordinate apparatus structure and high energy consumption are all directly related to the spray system. The nonuniformity of the spray system results from the spray pattern wherein the areas of the plate surface impinged by the rod like jet sprays cool faster than the plate surfaces in the gaps between the sprays. This is amplified when the gap areas of the top and bottom plate surfaces are directly opposite. The plate flatness problem results from the difficulties equating the cooling effect of the top and bottom sprays. The plate is cooled by two separate sprays diversely affected by gravity in which the top surface floods and the bottom sprays impinge the plate and fall away. This action creates unequal quenching uniformity and severity between the top and bottom plate surfaces. This is further complicated by the quench being divided lengthwise into three sections controlled by separate flow valves, by the random choking and blocking of the spray pipe orifices and the changes of water pressure with different water levels in the elevated water tank.
In retrospect, the reasons the plate and quench manufacturers never considered abandoning the spray quench system were primarily financial. The plate industry committed to the roller spray quench in the 1950's and the continuous plate heat treating lines operated at a profit for years. When the plate manufacturers encountered difficulties treating the leaner alloy plates in the 1960's all their efforts were made to improve the existing spray equipment. The U.S. Steel Corp. research study, showing the dip quench superior to the spray quench, was never published.
The continuous plate heat treating lines remained profitable because 80% of the plates heat treated were the air cooled normalized grades, by adding alloys to the quench and temper grades and the plate manufacturers tolerating the production delays caused by the blocking and choking of the spray pipe orifices.
The roller spray quench had no serious competitors but other plate quenches were developed for quenching plates on the rolling mill tables immediately after being controlled rolled. This came about after the development of the controlled rolling process which achieves a fine austenitic grain suitable for quenching.
A French company, USINOR in Dunkirk, developed the Rapid Accelerated Cooling, RAC, process in which the austenitic plate immediately after rolling is held flat between upper and lower rolls while traveling through a water tunnel. This process attempts total transformation and requires further tempering.
A Japanese company, Fukayame Works of Nippon Kokan, developed the On Line Accelerated Cooling, OLAC, process in which the plates immediately after rolling are cooled by laminar water flow on the top surface and water sprays on the bottom. The O.L.A.C. process does not attempt to produce the high strength steel grades which require a tempering heat treatment after quenching. The O.L.A.C. process partially quenches the carbon steel grades after controlled rolling to produce a bainite microstructure leaving the plate with sufficient retained heat to temper the bainite and aid in the plate leveling process. The purpose is to achieve the required physical properties in carbon steels with lower carbon content to improve the welding.
The overriding disadvantage of quenching plates on the rolling mill tables is the addition of a cumbersome operation with special requirements to the rolling mill which decreases rolling mill production.